CAISA'I'IOX ()|- I'LAXT DISEASES 



267 



the surl';ic(> lirout li. (Ichiliii was licnicdcd l)y 

 all ol" the noiiacid-t'asl cullurcs and hy the 

 two nonnuci-Dacrophilic acid-l'ayt forms. 'J'lic 

 13 microMcrophilic acid-last isolates ditl not 

 liquefy jj;ela(in. 



Typical potato scab, as distinguished from 

 superficial russetting of the surface, w as jjro- 

 duccd h\- all of the "dark brown ring" iso- 

 lates; none of tiie other isolates that were 

 tested caused typical scab on potato tubers. 

 Within the first group, ^•ariations occui-nni 

 in the amount of scab formed on the tubers, 

 though all produced some scab infection. The 

 particular cultures did not differ consistently 

 in type of scab lesion produced. In many 

 cases scab lesions \-arying from shallow de- 

 pressions to deep pits were found among the 

 replicated pots of one isolate, and frequently 

 within the same pot or on the same tuber. 



Superficial russetting was not considered 

 as typical of potato scab. It was more abun- 

 dant with certain isolates in the nonacid-fast 

 group. This russeting was not observed in 

 the tests of the "dark brown ring" group. 

 The correlations observed between patho- 

 genicity and the beha\'ior on milk held true 

 for a large number of actinomycetes. The 

 conclusion was reached that the ability to 

 produce typical lesions of potato scab was 

 correlated perfectly with the production of a 

 dark brown ring of surface growth on sepa- 

 rated milk. 



Afanasiev isolated se\'cn cultures of the 

 scab-producing organism from three differ- 

 ent types of scab: common, deep, and russet. 

 Indi\'idual potatoes were found to show two 

 or even all three forms of scab. The difference 

 in the kind of scab caused by cultures of ac- 

 tinomycetes was believed to be one of degree 

 of pathogenicity rather than of type of organ- 

 ism involved. Certain marked physiological 

 differences were found between the parasitic 

 and the saprophytic cultures. The parasites 

 were able to utilize sucrose and raffinose; 

 most of the saprophyt(\s were unable to 

 use these two sugars. The gi-owth of both 



types of cullurcs was similar on all media 

 conlaining dilTeicnl nil rogenous compounds, 

 with the exception of uica; I Ik; parasitic and 

 some of the saprophytic cultures failed to 

 grow on a medium to which 0.5 per cent of 

 urea was added. This was found to be due 

 to the toxicity of the ammonia produced as a 

 result of the decomposition of the urea. On 

 the basis of their physiological and biochemi- 

 cal properties, namely, ability to utilize su- 

 crose and raffinose, inhibition of growth by 

 ammonia, and ability to produce a melanin 

 pigment in a tyrosine medium, parasitic cul- 

 tures could be differentiated from ordinary 

 saprophytic actinomycetes. 



Finally, it may be of interest to report here 

 the recent work of Hoffmann (1958). Nu- 

 merous infection experiments were carried 

 out with twenty Streptomyces species, using 

 a munber of scab-susceptible potato \'arieties 

 in the greenhouse and under field conditions. 

 He found that S. scabies was the sole patho- 

 gen of potato scab; the same was true of 

 beet scab. He regarded the "parasitic" forms 

 of scab reported by other investigators as 

 saprophvtic organisms accompanying the 

 actual pathogen. 



Environment 



The nature of the soil in \vhich the pota- 

 toes are grown and the environment play 

 the greatest role in the production of scab. 

 The oiganic matter content of the soil, and 

 its degree of decomposition, the inorganic 

 structure of the soil, and the soil reaction 

 (Gillespie) are among the most important 

 factors in this connection. The environmental 

 conditions comprise moisture, temperature 

 (Jones et ai), and aeration. 



According to Dippenaar, the optimum 

 temperature for scab infection, as deter- 

 mined by the average number of lesions per 

 tuber, was found to be betw^een 17 and 21 °C. 

 The optinunn temperature for the develop- 

 ment of the disease was about the same as 

 the one best suited for the host, and did not 



